It is well known that the charging current applied to a lead acid battery should be decreased as the battery comes up to full charge. Generally, the decrease has been empirical. Hydrogen gas is evolved as a lead acid battery is charged. This has been suggested as the basis of a method of controlling the charging rate. Prior workers have suggested sensing gas pressure or measuring the noise or vibration caused by the gas bubbles. Those ideas have not been commercialized for obvious reasons.
There is a commercially available smoke/gas detector gased on N-type sintered SnO.sub.2. When combustable or reducing gases are absorbed on the sensor surface, a marked decrease of resistance occurs. The decrease is so sharp (it is exponential) that the control is obviously suited to use as a triggering control for various alarms and on-off devices.